Inference-based extension activation may be provided. Extensions, add-ins and/or plug-ins activate during reading, editing or composing of documents such as e-mails, calendar items, contacts, tasks, documents, spreadsheets, etc. In conventional systems, the activation may occur on demand or automatically based on the content of the document in question. In some situations, the basic conditional activation using content substring matching or pattern-based content matching is insufficient for activation based on heuristic matching of document content and/or data from external sources.